Replication of T4 DNA in vivo requires several proteins encoded by the phage DNA including the gene 43 DNA polymerase, the gene DNA unwinding protein, the gene 44 and 62 DNA-dependent ATPase, the gene 45 protein, and the gene 41 single-stranded DNA-requiring nucleotidase. We Have purified these proteins and have shown that they stimulate DNA synthesis in vitro with the T4 DNA polymerase with both native and single-stranded templates. We are presently studying the mechanisms by which chains are initiated with circular single-stranded templates with these proteins, as well as the mechanisms by which they accelerate the elongation of the primer and unwind duplex templates. Using a new screening procedure, we have identified a temperature-sensitive E. coli mutant (mut3) with a high mutation frequency (mutator) which maps at the dnaE locus encoding DNA polymerase III and complements other dnaE mutants in vivo. DNA polymerase III activity from this mutant is not thermolabile in vitro, but the secific activity for polymerase III decreases after growth at nonpermissive temperatures. The increased mutation frequency (about 100-fold) appears to result from incorporation of incorrect nucleotides during DNA synthesis rather than to induction of the "error prone" repair system. BIBLIOGRAPHIC REFERENCES: Gillin, F.D., and Nossal,N.G.: Control of mutation frequency by bacteriophage T4 DNA polymerase. I. The CB120 antimutator DNA polymerase is defective in strand displacement. J. Biol. Chem. 251: 5219-5224, 1976. Gillin, F.D., and Nossal, N.G.: Control of mutation frequency by bacteriophage T4 DNA polymerase. II. Accuracy of nucleotide selection by the L88 mutator, CB120 antimutator, and wild type phage T4 DNA polymerases. J. Biol. Chem. 251: 5225-5232, 1976.